Objective: The primary objective of this study is evaluate the effects of a fat-modified diet on serum sex hormones during adolescence. Secondary objectives are to assess associations of age, anthropometric measures, physical activity, and nutrient intake with levels of these hormones, and in girls, with age at menarche, menstrual cycle length, and frequency of anovulatory cycles. Background: Adolescent diet, especially fat intake, has been implicated in the etiologies of breast and prostate cancers. Limitations of adults' abilities to recall adolescent diet reduce the usefulness of case-control studies to evaluate these associations. Because of the duration of followup required, cancer is an unrealistic endpoint for cohort studies and clinical trials initiated during adolescence. Studying the effects of diet during adolescence on hormones that have been associated with breast and prostate cancers is a realistic alternative. Methods: This study is being conducted ancillary to the Diet Intervention Study in Children (DISC), a multicenter clinical trial sponsored by the National Heart, Lung, and Blood Institute. A total of 663 boys and girls, 8-10 years old, with elevated LDL-cholesterol were recruited into DISC beginning in 1988. Children were randomized to either a fat-modified diet providing less than 28% of calories as fat or to routine care. Progress: DISC is currently in its 8th year of followup. Prior to initiation of the ancillary study, a hormone feasibility study was conducted. This study has been completed and a publication is in press. Hormone assays are complete for visits through the 5-year followup visit, and statistical analyses are in progress.